Are some endosymbionts more prone to horizontal transfer and
recombination?
One explanation for the observed patterns could be the relative ease
with which supergroup A Wolbachia and Cardinium can
undergo horizontal transfer and recombination as compared to supergroup
B Wolbachia and Arsenophonus . This essentially means that
the former two endosymbionts would encounter previously existing
bacterial infections within their hosts which would increase the
opportunity for recombination among the pre-existing and the new
bacterial strains. Recombination would then create newer allele
variants. This is indeed borne out by the results in table 1 which
depicts the number of unique alleles found in this study among theWolbachia infections. In all about 84% (71 out of a possible 85
alleles) of the A supergroup infection are unique. Whereas, about 75%
(56 out of a possible 75 alleles) are unique in B supergroupWolbachia infections. Furthermore, as indicated in table 2, the
number of within supergroup recombination detected in the A supergroup
strains (8 instances) far outnumber the B supergroup Wolbachia ,
where none were detected. This is in spite of horizontal transfer of the
entire B supergroup ST’s (ST-541 and ST-559) to taxonomically unrelated
hosts (Table 1). An expected outcome of such pervasive horizontal
transfer and resulting recombination would have been an increase in
sequence diversity in the A supergroup strains, especially, if the
source of recombination had been infections outside the community. This
does not seem to be the case as the A supergroup infection show less
than expected pairwise distance (2.67%) when compared with the B
supergroup infections (4.17%). This indicates that the sources of
recombination must be from infections within this community. In other
words, the standing sequence variation of the A supergroup infections is
being partitioned across the community-wide arthropod taxa into newer
recombinants with resulting increase in allele diversity but not overall
sequence divergence. Moreover, what follows from this relatively low
pairwise divergence of the A supergroup infections is that this
horizontal transfer and recombination must have been recent or rapid
enough for any post-recombination sequence variation to accumulate. This
indicates that the A supergroup infections are either better at
horizontal transfer across the community or are presently undergoing
such rapid transfers as has been suggested by Werren et al (1995). On
the other hand, the B supergroup Wolbachia infections show
relatively diverged strains with low rates of recombination indicating
much stable infections. Since, little is known about the biological
characteristic of the different Wolbachia supergroups, other than
sequence divergence, it is difficult to speculate whether there are
supergroup specific effects on their hosts. For example, it is not known
whether any supergroup infections exclusively infect any specific
arthropod taxa or whether any supergroup make hosts more prone to
horizontal transfer? Therefore, we concentrate on specific trophic
interactions of the hosts themselves and try to explain why supergroup A
infections show such extensive horizontal spread.